1. Field of the Invention
The present invention relates to a method and apparatus for coating a substrate with a coating material by using a coating die and an apparatus for supplying the coating die with the coating material. Particularly, the present invention relates to a method and apparatus for applying the coating material with a constant thickness onto a thin substrate by using the coating die. Further, the present invention relates to an apparatus for supplying the coating material. The present invention is applicable to a coating system having the coating die, but not limited thereto.
2. Description of the Related Art
Both Japanese Patent Laid -Open Publications Tokkaihei 4-61955 and Tokkaihei 1-135565 disclose a method for applying a photo-resist onto a glass plate. Typically, this method is referred to as spin-coating. With this spin-coating, the glass plate is horizontally supported on a rotatable spin chuck. The photo-resist is poured onto an upper central portion of the glass plate. Then, upon rotation of the spin chuck holding the substrate, the poured photo-resist flows outwardly across the entire surface of the glass plate due to a centrifugal force created by the rotation.
The spin coating, however, can only retain a small part of the poured photo-resist (i.e., only about five percent thereof) on the glass plate, and the majority of the photo-resist (i.e., 95 percent thereof) is wasted without being recycled. This waste makes spin-coating significantly expensive.
Japanese Patent Laid-Open Publication Tokkaisho 56-159646 discloses another coating method in which a coating die has a distribution slot for distributing a coating material therefrom. According to this coating method, simply by moving the coating die along a surface of a glass plate, all the photo-resist distributed from the nozzle will be applied onto the surface of the glass plate. This is more economical than spin-coating.
However, most glass plates have three-dimensional deformations (e.g., twisting and curving) and uneven thicknesses. Likewise, a table for supporting the glass plate includes such three-dimensional deformations. A film, having a thickness of only 10 .mu.m or less, coated by the coating die on the substrate supported by the table will have a striped pattern with thin and thick areas due to gap variations between the coating die and the substrate.
To overcome this problem, Japanese Patent Laid-Open Publication Tokkaihei 7-328513 discloses a method for controlling a coating die. With this method, during coating, gaps (actual gaps) between a nozzle of the coating die and each successive portion of a substrate are pre-measured by a range sensor mounted on the coating die. Using the measured gap values, the coating die is moved towards and away from the substrate in order to keep the actual gap constant.
In this approach, however, two processes are required: one process for measuring the gaps between the nozzle and the substrate; and another process for calculating deviations between the successive gaps (i.e., measurements) and a reference gap predetermined for forming a coating film of specific thickness. Also, these processes must be done simultaneously during coating. However, the latter calculation process requires a considerable amount time, which prevents the calculations from matching the movement of the coating die. Therefore, this approach can only be applied when the coating speed is lower than the calculation speed, and results in an unacceptable delay in coating.
Japanese Patent Laid-Open Publication Tokkaihei 5-185022 discloses another coating method. According to this method, a thickness for each successive portion of a member to be coated (a metal plate) is pre-measured by a sensor at a measurement station at an upstream side of a coating station with respect to a traveling direction of the metal plate. Using the measured values, an applicator (e.g., coating die or blade) is moved towards and away from the metal plate. In this method, however, the measurement station is spaced apart from the coating station. Therefore, if there exists an error or height difference of about several micrometers with respect to a direction perpendicular to a major surface of the metal plate between a first surface portion for supporting the metal plate at the measurement station and the second surface portion for supporting the same at the coating station, it is impossible to measure such a height difference. This in turn prohibits a correction for the height difference between a first surface portion at the measurement station and a second surface portion at the coating station. As a result, a gap between the applicator and the metal plate at the coating station can not be adjusted to a predetermined reference gap even by driving a motor to move the coating device towards and away from the metal plate with an aid of a controller.
A device for supplying the coating die with the coating material has been known. This device has a reservoir for accommodating the coating material and a feed pipe fluidly connected between the reservoir and the coating die. The feed pipe is provided with a pump for feeding the coating material into the coating die and a filter so that the coating material is supplied from the reservoir to the coating die. However, when using a gear pump and a volute pump as the pump, a number of small bubbles, each having a diameter of about 0.01 mm, are invariably mixed into the coating material which is discharged from the pump. The small bubbles hardly affect the thickness of the coated film when the feed pipe has an inner diameter of 5 mm or more.
If, however, the feed pipe has an upwardly curved or bent portion, the small air-bubbles can group together to grow into a relatively large bubble having a diameter of 1 mm or more. The large bubble tends to contract and expand while traveling in the pipe due to changes in pressure from the pump. Then, if the feed pipe has an inner diameter of about 5 mm or less, the resultant contraction and expansion leads a pressure variation in the coating material in the feed pipe. This in turn results in unevenness in the resultant coating. Particularly, this pressure variation will be problematic when a finished thickness of the coating is 10 .mu.m or less and when a thickness variation of .+-.5% or less is required for the resultant coating in a wet condition.